WASHINGTON (Reuters) - Eight to 10 Republican U.S. senators have serious concerns about Republican healthcare legislation to roll back Obamacare, moderate Republican Senator Susan Collins, who opposes the bill, said on Sunday.

The Senate, which is delaying its consideration of the bill while Arizona Republican Senator John McCain recuperates from surgery to remove a blood clot, will take it up as soon as all senators are available, Senator John Cornyn, the second-ranking Republican senator, said.

McCain's absence casts doubt on whether the Senate would be able to pass legislation to repeal parts of the Affordable Care Act, Democratic former President Barack Obama's signature legislative achievement, commonly known as Obamacare.

Collins is one of two Republican senators who have already said that they would not even vote to open debate on the latest version of the bill released on Thursday, meaning one more defection from the Republican ranks could kill it..

Republicans control the Senate by a 52-48 margin. With the Democrats solidly opposed to the legislation, the Republicans can only pass the bill if all their other members back it and if Republican Vice President Mike Pence casts his tie-breaking vote in favor.

A Washington Post-ABC News poll published on Sunday showed Americans preferred Obamacare by a 2-1 margin. Approaching six months in office, Trump’s overall approval rating has dropped to 36 percent from 42 percent in April.

While Collins said that she did not know if the legislation would ultimately pass, she said as many as 10 Republicans have doubts about it.

BAD STRATEGY

"There are about eight to 10 Republican senators who have serious concerns about this bill," Collins told CNN's "State of the Union" program, faulting the bill for its major cuts to the Medicaid government health insurance program for the poor, which she said would harm rural hospitals and nursing homes.

"I don't know whether it will pass, but I do know this, we should not be making fundamental changes in a vital safety net program that's been on the books for 50 years - the Medicaid program - without having a single hearing to evaluate what the consequences are going to be," she added.

Republican Senator Rand Paul also reiterated his opposition to the bill, which he described as "terrible" because it retained many of the Obamacare taxes and subsidies.

"The current system is terrible," Paul said on Fox News Sunday. "I don't think Republicans should put their name on this. It is a bad political strategy and it will not fix the problem."

The bill unwinds Obamacare's Medicaid expansion over three years, from 2021 to 2024. But it goes beyond repealing Obamacare by imposing drastic cuts to Medicaid that deepen in 2025.

McCain, who plans to stay in Arizona this week after a procedure to remove a 2-inch (5-cm) blood clot from above his left eye, has expressed concern about the healthcare bill but has not said how he would vote.

"We need Senator McCain in more ways than one. As soon as we have a full contingent of senators we will have that vote," Cornyn, of Texas, told NBC's "Meet the Press" program.